Rename fail! to panic!

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]

src/libstd/lib.rs

@@ -91,7 +91,7 @@
 //! Finally, the [`prelude`](prelude/index.html) defines a
 //! common set of traits, types, and functions that are made available
 //! to all code by default. [`macros`](macros/index.html) contains
-//! all the standard macros, such as `assert!`, `fail!`, `println!`,
+//! all the standard macros, such as `assert!`, `panic!`, `println!`,
 //! and `format!`, also available to all Rust code.
 
 #![crate_name = "std"]
@@ -261,7 +261,7 @@ mod std {
     pub use io; // used for println!()
     pub use local_data; // used for local_data_key!()
     pub use option; // used for bitflags!{}
-    pub use rt; // used for fail!()
+    pub use rt; // used for panic!()
     pub use vec; // used for vec![]
 
     // The test runner calls ::std::os::args() but really wants realstd